In holographic data storage digital data are stored by recording the interference pattern produced by the superposition of two coherent laser beams, where one beam, the so-called ‘object beam’, is modulated by a spatial light modulator and carries the information to be recorded. The second beam serves as a reference beam. The interference pattern leads to modifications of specific properties of the storage material, which depend on the local intensity of the interference pattern. Reading of a recorded hologram is performed by illuminating the hologram with the reference beam using the same conditions as during recording. This results in the reconstruction of the recorded object beam.
One advantage of holographic data storage is an increased data capacity. Contrary to conventional optical storage media, the volume of the holographic storage medium is used for storing information, not just a single or few 2-dimensional layers. One further advantage of holographic data storage is the possibility to store multiple data in the same volume, e.g. by changing the angle between the two beams or by using shift multiplexing, etc. Furthermore, instead of storing single bits, data are stored as data pages. Typically a data page consists of a matrix of light-dark-patterns, i.e. a two dimensional binary array or an array of grey values, which code multiple bits. This allows to achieve increased data rates in addition to the increased storage density. The data page is imprinted onto the object beam by the spatial light modulator (SLM) and detected with a detector array.
Typical laser diodes emit elliptical beams with Gaussian intensity distribution. This leads to inhomogeneous power distribution on the spatial light modulator that produces the data pages in a holographic storage system. To improve the signal to noise ratio of the data page, and thereby the capacity, a homogeneous intensity distribution is preferable. The elliptical distribution can be eliminated easily, for example by using a prism pair. The Gaussian distribution can be reduced by special optics with aspheric surfaces. However, beam-shaping optics are expensive and extremely sensitive to misalignment.
For example, U.S. Pat. No. 6,654,183 describes a system for converting a substantially non-uniform optical input beam, such as a Gaussian beam, to a substantially uniform output beam. For this purpose the system has two optical elements with aspheric surfaces arranged in either a Keplerian or Galilean configuration.